Orthogonal Frequency Division Multiplexing (OFDM) has been accepted as a mature technology for wireless broad-band communication links. Its design as a multi-carrier system, allows the support of high data rates while maintaining symbol durations longer than the channel's dispersion time. Thus, OFDM modems can have reliable high data rate transmission in time-dispersive or frequency-selective channels without the need for complex time-domain channel equalizers. OFDM can also provide high spectral efficiency by taking advantage of the adaptive nature of the multi-carrier modulation. Information feedback of the channel conditions is used for allocating and adapting the modulation load for each sub-carrier in order to maximize the channel throughput. Such a prior art concept, is effective when channel conditions vary slowly. Another prior art approach, provides suppression of narrow-band interferers by using a hidden Markov model filter for detecting and estimating interference in the frequency domain.
Related prior art methods, used in multi-user communication, are known as multi-user OFDM, multi-carrier or multi-tone CDMA. These methods are the results of combining the OFDM with CDMA. The CDMA (binary) spreading sequences are used for distinguishing and separating the user transmissions while OFDM is used to enhance the link performance of each transmission in the presence of interference (from the other users) and channel impairments. A multi-user OFDM or multi-carrier CDMA system may be a synchronous one if its transmission is in the downlink (point-to-multipoint) or an asynchronous one if its transmission is in the uplink. Also, a multi-user OFDM or multi-carrier CDMA system may either be a bandwidth expanding (spreading the spectrum) one or a non-bandwidth expanding (not spectrum-spreading) one. The non-bandwidth expanding multi-carrier CDMA systems are based on the method of orthogonal code division multiplexing (OCDM), or multi-code CDMA (MC-CDMA) in which a serial-to-parallel converter is used to provide parallel outputs that are “spread” to the original input rate by means of orthogonal sequences.